1. Field of the Invention
The present invention relates to an image forming apparatus which performs color registration.
2. Description of the Related Art
In recent years, there has been an increasing market demand for multifunctional peripherals including a plurality of output terminals such as a copier, a printer, and a facsimile machine. In those output terminals, electrophotographic image forming apparatus are used widely.
In the case where an image forming portion is divided into a plurality of stations for respective colors in a color image forming apparatus, an image position formed in each station may be displaced in a process proceeding direction (hereinafter referred to as “sub scanning direction”) or a longitudinal direction (hereinafter referred to as “main scanning direction”). This is called color misregistration, and the occurrence of the color misregistration leads to degradation in image quality. Factors for causing the color misregistration mainly include the deformation of an exposure device caused by a temperature change and the variation in a light irradiation position on the surface of an image bearing member resulting from the deformation. Although the color misregistration is within a certain range due to the accuracy of constituent components, color misregistration of about tens of to hundreds of μm may occur, depending on the deformation of a main body and an exposure device resulting from a rise in temperature in image formation.
In order to correct the color misregistration, a control mechanism configured to form a color registration pattern on the surface of an image bearing member, read the formed pattern with an optical sensor (registration detecting sensor), and perform color registration is mounted on a conventional product. For example, Japanese Patent Application Laid-Open Nos. H01-142676 and H05-188697 disclose a configuration of changing timing for performing color registration depending on a temperature change detected by a temperature sensor of a main body of an image forming apparatus, and a configuration of changing timing for performing color registration depending on an accumulated time period from power-on of the image forming apparatus.
In the conventional image forming apparatus, the color registration is performed after it becomes possible to form an ordinary image in usual image formation, and hence, first copy output time (FCOT) increases along with the color registration. The FCOT refers to a time period required from the start of an image forming process to the output of a transfer material on which an image is formed first. It is also important for increasing the speed of image formation to shorten the FCOT which increases along with the color registration.